Thursday, October 28, 2010

Scat-Firing Caterpillars Elude Predators

John Pickrell in Englandfor National Geographic News

May 16, 2003

Several species of caterpillars have developed an interesting system for waste disposal; they fire their fecal pellets a distance of up to 40 times their body length away from their homes, at a speed of 4.2 feet (1.3 meters) per second. The equivalent distance for a 6-foot-tall (1.8 meter) human would be around 240 feet (73 meters).

Scientists have long speculated on the evolutionary factors that would favor the development of this extraordinary behavior.

"While studies of foraging have been a cornerstone of ecological research, analogous issues related to defecation have received much less attention," said Martha Weiss, an ecologist at Georgetown University in Washington, D.C.

A lot of animals distance themselves from their waste, usually for reasons of hygiene. Some nestling birds, for example, package waste into mucilage-coated sacs ready for convenient disposal by the adults in the nest. Other animals are known to use scat for surprising purposes; larval tortoise beetles pile fecal shields on their backs to protect them from predators. Some caterpillar species climb onto silk strings decorated with fecal pellets, which are known as frass. The frass helps protect them from ants.

In Weiss's research of scat-launching caterpillars, she found the first experimental evidence that the adaptation serves to protect the caterpillar larvae from wasps and other predators.

Scat-Throwing Launch Pad

Skipper butterfly caterpillars are able to fire frass pellets by pumping up blood pressure directly under an anal "launching pad" on which extruded fecal pellets rest.

"It's the equivalent of the mechanism involved in flicking a pea," said Stanley Caveney, a biologist at the University of Western Ontario in London, Canada.

Caveney discovered the "scatapulting" mechanism, leaving scientists with questions about its purpose.

One explanation favored by scientists was that the caterpillars are just good housekeepers and keep their silk-stitched leaf shelters spotless for hygienic purposes.

Weiss's research provides the first experimental evidence for the predator avoidance idea.

"Until now, there has been no direct evidence to suggest the idea that fecal firing behavior in caterpillars helps them to avoid being caught by [predators]," Caveney said.